Conventional gas turbine engines generally operate on the principle of compressing air within a compressor section of the engine, and then delivering the compressed air to the combustion section of the engine where fuel is added to the air and ignited. Afterwards, the resulting combustion mixture is delivered to the turbine section of the engine, where a portion of the energy generated by the combustion process is extracted by a turbine to drive the engine compressor. In multi-stage turbine sections, stators are placed at the entrance and exit of the turbine section, as well as between each turbine stage, for purposes of properly directing the air flow to each successive turbine stage. As a result, the stators are able to enhance engine performance by appropriately influencing gas flow and pressure within the turbine section.
Stators generally consist of an annular array of airfoils, or vanes, which are supported by a pair of concentric annular bands, all of which are preferably cast from a suitable high temperature material, such as a single-crystal nickel-based superalloy. It is generally impractical to form stators from a single casting, particularly those for use in larger engines, due to metallurgical constraints imposed during the casting process, as well as excessive thermal stresses created by nonuniform temperature distributions within the engine. As a result, stators are typically formed in segments as stator vane assemblies consisting of one or more airfoils positioned between an inner and outer band member. These vane assemblies are then individually mounted to the engine casing to form an annular array of stator vane assemblies, which is then located within the turbine section of the engine so that the airfoils project radially between an adjacent pair of turbine stages.
Various approaches have been proposed for constructing the stator vane assemblies, the most common approaches being illustrated in FIGS. 1 and 2. FIG. 1 depicts a two-piece vane assembly 48 which consists of a pair of vane castings 40 which are brazed together. Each vane casting 40 includes an outer band member 42, an inner band member 44 and an airfoil 46. The vane castings 40 are preferably single crystal or directionally solidified castings so as to enhance mechanical properties at elevated temperatures.
A disadvantage to this vane assembly construction is that for large vane castings 40 there tends to be recrystallization during the casting process, thereby altering the grain structure of the vane casting 40, particularly at the airfoil-to-outer band interface. Recrystallization occurs as the metal volumetrically shrinks during cooling in a mold which is conventionally made of ceramic. As the casting cools, the airfoil 46 contracts in length such that the outer and inner bands 42 and 44 contract toward each other, while contraction within the ceramic mold occurs to a much lesser degree. As a result, the mold serves as a restraint, preventing the outer and inner bands 42 and 44 from contracting towards each other as they would otherwise. As a result of this phenomenon, the high cast-in stresses which are created within the airfoil-to-outer band transition region of the vane casting 40 causes recrystallization in those regions, which adversely effects the mechanical properties of the vane casting 40, particularly at engine operating temperatures.
The primary alternative to the construction method of FIG. 1 is illustrated in FIG. 2. This vane segment is a four-piece assembly 50 consisting of an outer band 52, and inner band 54 and two airfoils 56, each of which are individually cast. Both the outer and inner bands 52 and 54 are provided with slots or openings 58 into which the ends of the airfoils 56 can be brazed in place to form the vane assembly 50. An advantage with the use of this construction method is that cast-in stresses are substantially avoided, reducing the likelihood of recrystallization.
However, a disadvantage with this approach arises from the vane assembly 50 being generally attached to the engine casing at the outer band 52 only, with the airfoils 56 and inner band 54 being essentially cantilevered into the engine's air stream. Consequently, the highest mechanical stresses in the vane assembly 50 occur at the airfoil-to-outer band interface which, in this instance, is a braze joint whose strength is inferior to that of an integrally cast interface. In this regard, the integrally cast construction of the vane assembly illustrated in FIG. 1 is superior.
From the above, it can be seen that vane assemblies taught by the prior art are generally either subject to recrystallization during the casting process, which is detrimental to the vane assembly's high temperature properties, or is subject to fatigue cracking and fracture as a result of mechanical stresses being concentrated at a braze joint.
Accordingly, it would be advantageous to provide an improved stator vane assembly whose construction overcomes the disadvantages of the prior art. Specifically, it would be desirable to provide a stator vane assembly composed of single crystal or directionally solidified cast components which are less prone to recrystallization during the casting process, and whose construction avoids maximum stress concentrations at braze joints between the cast components.